Are there any rules relevant to involuntary movement and effects other than Opportunity Attacks?

PHB p. 195 states that involuntary movement does not provoke opportunity attacks:

You also don’t provoke an opportunity attack when you teleport or when someone or something moves you without using your movement, action, or reaction.

Is there a general rule somewhere about involuntary movement and how it interacts with other effects?

The question arose at our table about what happens if a creature is dragged via the Thorn Whip spell across an area of Spike Growth. Does the involuntarily moving creature take the damage from the spikes as if he’d voluntarily moved through them?

Logically, yes, if you’re being dragged through spikes, you take damage from them as if you’d walked through them. But logically, if I drag you past my ally, my ally would be able to make an Opportunity Attack, likely more easily than if you were moving under your own power. Presumably that is not allowed for balance reasons rather than versimilitude reasons, so I wonder if the same should apply to other effects?

Can you take free actions during attacks of opportunity?

I have read some claim that free actions cannot be taken during attacks of opportunity. However,

Free Action

Free actions consume a very small amount of time and effort. You can perform one or more free actions while taking another action normally. However, there are reasonable limits on what you can really do for free.

Emphasis mine. Why would this exclude an Attack of Opportunity?

Speak

In general, speaking is a free action that you can perform even when it isn’t your turn. Speaking more than few sentences is generally beyond the limit of a free action.

I’ve seen references to speaking, particularly, “when it isn’t your turn”. I’ve always understood that to mean you can speak out of turn, like an immediate action, not that all other free actions must be on your turn.

For example, you can yell out a warning, even when it isn’t your turn, but you can’t drop prone unless you were otherwise taking another action.

Is there another rule this comes from? Is it in the Rules Compendium?

If you have the War Caster feat and cast Booming Blade as a reaction, does it count as an opportunity attack for the UA Blade Mastery feat?

The third benefit of the Blade Mastery feat (from Unearthed Arcana: Feats) says:

You master the shortsword, longsword, scimitar, rapier, and greatsword. You gain the following benefits when using any of them:

[…]

  • When you make an opportunity attack with the weapon, you have advantage on the attack roll.

The third benefit of the War Caster feat (PHB, p. 170) says:

  • When a hostile creature’s movement provokes an opportunity attack from you, you can use your reaction to cast a spell at the creature, rather than making an opportunity attack. The spell must have a casting time of 1 action and must target only that creature.

If you use your reaction to cast booming blade, does that count as making an opportunity attack as outlined by the Blade Mastery feat, since you are making a weapon attack as part of booming blade?

Long-limbed with reach. Polearm Master, Sentinel and Tunnel Fighter. Opportunity attack slow combo? [on hold]

So a Bugbear with a polearm has a 15ft reach(Long-Limbed) each creature who enters its reach gets attacked(Polearm Master) and they must stop if hit(Sentinel) they would have to take 3 turns to be able to attack the character or anyone by/behind them stopping every time they move 5ft(Tunnel Fighter)providing the attacks hit. This seems really OP.

Polearm Master:

When you take the Attack action and attack with only a glaive, halberd, or quarterstaff, you can use a bonus action to make a melee attack with the opposite end of the weapon. This attack uses the same ability modifier as the primary weapon. The weapon’s damage die for this attack is a d4, and the attack deals bludgeoning damage. While you are wielding a glaive, halberd, pike, or quarterstaff, other creatures provoke an opportunity attack from you when they enter the reach you have with that weapon.

Tunnel Fighter (Unearthed Arcana):

You excel at defending narrow passages, doorways, and other tight spaces. As a bonus action, you can enter a defensive stance that lasts until the start of your next turn. While in your defensive stance, you can make opportunity attacks without using your reaction, and you can use your reaction to make a melee attack against a creature that moves more than 5 feet while within your reach.

Sentinel:

You have mastered techniques to take advantage of every drop in any enemy’s guard, gaining the following benefits:

When you hit a creature with an opportunity attack, the creature’s speed becomes 0 for the rest of the turn. Creatures provoke opportunity attacks from you even if they take the Disengage action before leaving your reach. When a creature within 5 feet of you makes an attack against a target other than you (and that target doesn’t have this feat), you can use your reaction to make a melee weapon attack against the attacking creature